Applying sedimentary geochemical proxies for paleoenvironment interpretation of organic-rich shale deposition in the Sichuan Basin, China

• Published in: International journal of coal geology Vol. 163; pp. 52 - 71
• Main Authors: Zhao, Jianhua, Jin, Zhijun, Jin, Zhenkui, Geng, Yikai, Wen, Xin, Yan, Caina
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.06.2016
• Subjects: Biogenic silica; Geochemical proxies; Organic matter; Paleoenvironment; Sichuan Basin; Wufeng and Longmaxi Formations; Sichuan Basin; Organic matter; Wufeng and Longmaxi Formations; Paleoenvironment; Geochemical proxies; Biogenic silica
• ISSN: 0166-5162; 1872-7840
• DOI: 10.1016/j.coal.2016.06.015

Depositional environment highly affects the geochemical feature of sediments recording the information of paleoenvironment and its evolution. Geochemical data for Ordovician Wufeng Formation and Silurian Longmaxi Formation black shale deposition in the Sichuan Basin are presented and applied as proxies for deciphering paleoenvironment (detrital influx, redox conditions, paleoproductivity) and providing insight to paleoenvironmental conditions responsible for organic carbon accumulation. These data suggest that siliceous shale in the Wufeng and Longmaxi Formations reveals high TOC content, whereas silty, argillaceous, and limey shale display relatively low TOC contents. Major element and petrological evidences indicate that most of quartz in siliceous shales is biogenic origin (as shown by excess silica concentrations); therefore, Si is an unreliable indicator of detrital influx. The similar distribution of detrital influx proxies (Ti/Al, Ce, Hf, La, Nb Th and Zr) suggests a rather homogeneous coarse-grained detrital fraction supply. Redox proxies (Mo, Th/U, Ni/Co, V/Cr, and V/(V+Ni)) indicate that siliceous shales were deposited in a relatively dysoxic/anoxic environment, whereas the argillaceous, silty, and limey shales were deposited in a relatively oxic environment. Barium was solubilized under reducing conditions and cannot be applied confidently as a paleoproductivity indicator in high-TOC siliceous shales. Productivity proxies (Cu/Al, Ni/Al, excess Si) suggest that paleoproductivity was high during siliceous shales deposition, and weak during argillaceous and limey shales deposition. Our data suggests that high paleoproductivity and dysoxic/anoxic conditions controlled organic matter accumulation in Wufeng and Longmaxi shales. •Multiple sedimentary geochemical proxies were investigated for paleoenvironment interpretation.•Biogenic silica make silica or Si/Al ratios an unreliable indicator of clastic influx.•The low Batot and Babio concentrations does not suggest the lower paleoproductivity.•Anoxic conditions plays a significant role in organic matter accumulation.